The use of specialized ink formulations to form thick films having various functions on suitable substrates in the construction of multilayer integrated circuit structures is well known in the art. Such technology is of increasing interest in the fabrication of very dense multilayer circuit patterns on various substrates for a wide variety of applications in the electronics industry.
Significantly improved substrates for the fabrication of such circuits are disclosed and claimed in Hang et al., U.S. Pat. No. 4,256,796, issued Mar. 17, 1981, the disclosure of which is incorporated herein by reference. The Hang el al. substrates are comprised of a metal core coated with an improved porcelain comprised of a mixture, based on its oxide content, of magnesium oxide (MgO) or mixtures of magnesium oxide and certain other oxides, barium oxide (BaO), boron trioxide (B.sub.2 O.sub.3) and silicon dioxide (SiO.sub.2). The preferred metal is steel, particularly low carbon steel, which may be coated with various other metals such as, for example, copper. The porcelain compositions are applied to the metal core and fired to provide a partially devitrified porcelain coating thereon. The coating has a very low viscosity at its initial fusion point and then almost instantaneously obtains a high viscosity due to devitrification. The fired coatings, which are preferred for hybrid circuit applications, have a deformation temperature of at least 700.degree. C. and a high coefficient of thermal expansion of at least about 110.times.10.sup.-7 /.degree.C.
While the porcelain-coated metal substrates of Hang et al. represent a significant improvement over previously known substrate materials, they are disadvantageous only in being incompatible or poorly compatible with commercially available thick-film inks. For example, a resistor ink containing a significant amount of lead oxide, which is conventionally used in air-fireable inks, is a flux for the Hang et al. substrates. During firing, the glass in such an ink will drain to the substrate, pass therein and become diffused throughout to varying concentrations. This leaves the functional particles on the surface in a glass deficient state and substantially changes the makeup and properties of the substrate. The migration of the glass into the substrate produces very low resistance values and substantially increases the possibility that the ink can chip or flake off the surface. The resistance values abruptly decrease because the glass acts as an insulating barrier for the conductive functional ingredient, i.e. ruthenium dioxide. When the glass migrates into the substrate, the functional ingredient particles are drawn closer together without an insulator. This therefore significantly raises conductivity and, conversely, lowers resistance to unacceptably low levels. Air-fireable conductor and resistor inks compatible with the Hang et al. substrates are provided in accordance with this invention.